You know your child inside and out, which is why her strange behavior is worrying you.
Perhaps it's nothing at all, but doubt and fear gnaw at you nevertheless.
This is a common occurrence for parents, whose toddlers are not able to process and communicate their malaise.
As usual, my preface is that only a qualified physician will be able to fully assuage you fears, so this reference is meant solely as a broad guideline about what to look out for.
Please remember that only a doctor will be able to rule out or diagnose anything!

• Sudden and uncharacteristic lack of appetite
• Limpness and unresponsiveness
• A different tenure of crying (this may seem a bit obvious: examples include moaning, mewling or wailing)
• Your baby looks unusually pale
• Pitting or discolored skin (look for other signs and causes of dehydration: fever,vomiting or diarrhea)
• Fevers (call a doctor if you are facing a persistent fever of over 102 F)
• Drooling
• A lump on the fontanelle (forehead)
• Rapid breathing (possible hyperventilating)
• General uncharacteristic irritability
• Spreading of rashes (more so if accompanied by fever)
• Ear problems (look for ear discharge -- generally your child may even be unresponsive to sounds)

These guidelines are quite general, and in many cases the appearance of one of more symptoms listed may mean very little and may be considered normal.
More serious scenarios and symptoms may require emergency care.
Contact a doctor immediately if you run into the following symptoms:

• Continuous bleeding and large cuts
• Problems related to breathing
• Large burns (is your child suffering from shock?)
• Blue Lips
• Haematuria (blood in urine)
• Head injuries